In an aircraft, it is necessary to provide for means of evacuation of the occupants of the plane in emergency situations, particularly in the event of a crash in which the urgency of the situation and the deterioration of the aircraft prevent the use of the usual means. Each area must be treated separately. The cabin and the cockpit are two distinct areas.
The cockpit must therefore have its own means of evacuation.
The requirement in this regard as described by the regulations states that:                a single exit is sufficient if it is located on the centre line of the aircraft,        two exits are necessary if they are on both sides of the aircraft,        an emergency exit of the axial ventral type is not acceptable, as a landing with the retracted undercarriage blocking this exit means it is incapable of use.        
Today, the solutions currently used in a cockpit are either to use opening panes arranged to the left and right of the pilots or to use an evacuation hatch installed in the upper part of the cockpit, generally rearward of the pilots.
In addition to the safety requirement, there is a need for comfort. Although generally not mandatory, it is desirable when on the ground to offer natural ventilation in the cockpit, regardless of the existence of dedicated air conditioning in the cockpit.
The aforementioned solution consisting of two opening lateral panes generally addresses this problem satisfactorily, to the extent that the opening and closing thereof are carried out simply and reversibly. It is also possible to provide lateral openings of the single-use windscreen pane type for evacuation. Production is simpler in this case, but the comfort function previously described is not available.
Moreover, using opening lateral panes has the drawback in particular that it requires space to be available behind them to allow translational movement in order to clear the opening sufficiently; or else, due to duplication of the requirements for each pilot—the fact of having two required openings gives rise to a multiplication of the corresponding opening mechanisms for example—production in this way is more expensive in terms of weight and time, in comparison for example with a single upper opening constituted by a hatch, for example made of metal.
However, although such a hatch provides a single emergency route as prescribed by the regulations, it may be desirable to have a central exit with more convenient access, and in any case such a hatch does not allow the comfort associated with natural ventilation to be provided.